Poly-alpha-olefins traditionally have been catalyzed by well-known multi-site catalysts including Ziegler-Natta type catalysts such as titanium chloride. While such catalysts are useful for producing resins or polymers of alpha-olefins, including polypropylene, they produce polymers with relatively broad molecular weight distributions or polydispersity which will include significant fractions of polymer material with both higher and lower molecular weight than the average or nominal molecular weight of the polyolefin polymer.
The high molecular weight fraction included in the traditionally catalyzed polymer is likely to cause processing difficulties for the maker of polypropylene fibrous or fiber-containing products. While not wishing to be bound by theory, it is believed that the high molecular weight species of polypropylene which are found in such reactor-grade polymers, will contribute significantly to the melt strength of the molten polypropylene catalyzed, thus diminishing the processibility of the polymer, particularly in the field of fiber manufacture. The high melt strength of multi-site or other traditionally produced molten polypropylene will generally form fibers by a melt spinning process but will yield comparatively large diameter fibers when run at economically practical production rates. Such fibers may feel stiff or coarse when compared with other fibers, particularly natural fibers or fibers of a smaller diameter or lower denier.
Conceptually, it is easy to visualize why it is desirable to reduce the concentration of high molecular weight polypropylene species within a particular nominal or average molecular weight production batch. When it is considered that the longer polymer chains tend to be conformationally more bulky, coiled, or otherwise entangled, it can be understood that it becomes more difficult to move those particular species within the polymer. The fact that these high molecular weight chains are longer contributes to the need for higher processing temperatures and the high strength of molten polypropylene. From a kinetic standpoint, it becomes more difficult to add sufficient energy to these molecules to move them. Once those higher molecular weight species become molten they generally become more prone to entanglement due to their size. It is the presence of this significant fraction of the higher molecular weight species which causes numerous problems in the high speed production of fibers, in the instance of melt spinning, in the case of, for example, melt blowing or spun bonding, and subsequent formation of quality fabrics.
Some of the problems which may be created or at least exacerbated by the high molecular weight fraction include the need for higher processing temperatures which are necessary to reduce inherent melt strength and viscosity and cause the higher molecular weight chains to move. This will require higher energy input to move the polymer, which contains high molecular weight species, through the extruder or other processing equipment. All of this leads not only to higher wear on materials handling equipment, including extruders, resulting from the higher energy input necessary, but also likely degradation of polymer and polymer product properties.
For fibers, commonly, non-homopolyethylene higher polyolefins having the significant fraction of high molecular weight species will process with some difficulty. As mentioned, also, in copending U.S. Ser. No. 08/066,737, from which this application derives priority as a continuation-in-part, aside from the general difficulties encountered in processing this material additional problems are encountered with fiber processing. These include difficulty in making fine fibers at practical production rates which derives from the difficulty in extending the molten polymer fibril. This is associated with high melt strength of molten polyolefins, particularly polypropylene which includes high molecular weight species. High melt strength also leads to difficulty in forcing the molten resin through a small fiber-forming orifice. Within that restriction, the high molecular weight molecules will cause significant drag and diminish flow. Those same molecules will also cause significant die swelling of the polymer fibril upon its exit from the fiber-forming orifice due to their inherent tendency toward elastic response with recovery of their conformational bulk. It is this same fraction of high molecular weight molecules which will cause the molten polypropylene to have significantly increased melt strength over what should be expected at the nominal molecular weight. The net effect is that, again, the high molecular weight fraction will require higher melt temperatures and higher energy input into the machinery, which will create higher wear rates caused by higher pressures needed to force the material through the fiber-forming orifices. The necessarily high temperatures and pressures will also shorten the life of the fiber-forming die or spinerette.
Along with these processing difficulties for fiber manufacturers, the fibers resulting from traditionally produced polypropylene will tend to be thick, due to the melt strength of the molten resin. Such fibers will lead to formation of fairly coarse, boardy feeling fabrics which will not be stretchy or forgiving at points of flexion if worn as a garment. This coarseness and lack of "give" in such poly-alpha-olefin fabrics limit their use in garments and other applications where a pleasant feel or "hand" is desirable.
It is the stiffness of fibers and boardiness of their resulting fabrics produced from the traditionally catalyzed polyolefins, particularly polypropylene, which has led to some complicated post-formation processes. For example, Kobayoshi et al., describe, in U.S. Pat. No. 5,078,935 a mechanical creping step which puts crimps in the fibers and fabric after fabric formation to effectively make the fabric somewhat stretchy and less restrictive when used as a garment.
Significant advances were made in the 1970's in the field of post-reactor treatment of polypropylene to enhance processability. Most of these post-formation or post-reactor processes involve some sort of molecular chain scission of the polymer molecules. Such scission or molecular cleaving is normally accomplished through the treatment of polyolefins, particularly, polypropylene, with heat and oxygen, or a source of free radicals such as organic peroxides. For background purposes, some of these techniques are described in U.S. Pat. Nos. 3,608,001; 3,563,972; 3,862,265; and 3,898,209.
Timmons et. al. teach, in U.S. Pat. No. 5,188,885, formation of nonwoven fabric laminates using polypropylene. As described, the isotactic polypropylene which is used by Timmons is of low crystallinity, or low isotacticity. Further, Timmons describes use of Exxon Polymer Grades 3125 and 3214 for their formation of fabrics. It is clearly stated that the 3214 grade has been peroxide treated to reduce the melt viscosity by molecular scission. While it is not stated, the fact that the 3125 grade is a peroxide treated grade of polypropylene must be recognized. Post-reactor viscosity reduced polypropylene was used for half of the fabric laminates. The other half used an ethylene copolymer, not a poly-alpha-olefin.
Such post-reactor processing has the potential to cause molecular cleaving or general degradation of all polymer molecule chains within the polymer, thereby dramatically reducing the nominal molecular weight and the molecular weight distribution of the polymer. In light of the fact that the larger polyolefin molecules have more potential sites for oxidative degradation or scission, the fraction of high molecular weight species will be significantly reduced upon the exposure of the polymer resin to such post-reactor treatment.
The post-reactor treatment involving oxidative scission, as discussed earlier, has served a useful purpose for fiber makers. Such scission offers similar benefits to producers of fibers and fibrous products including those made by melt-spinning, melt blowing or spunbonding processes as it would for most other end-use product producers. These include reduced overall viscosity, shifted molecular weight distribution, reduced nominal molecular weight, and significantly reduced fractions of high molecular weight species.
Coupled with the benefits which are derived from the post-reactor oxidative scission treatment of polypropylene, however, are some significant drawbacks. While such a treatment of polypropylene does indeed significantly reduce the fraction of high molecular weight species which are present, it also dramatically increases the fraction of low molecular weight species in the polymer. The presence of these low molecular weight species causes various difficulties for the fiber manufacturer. Since the lower molecular weight species tend to be more mobile they also tend to become airborne or volatile during melt processing.
This volatility causes difficulty such as an apparent smoking from the material at high temperature when it is not contained, as when it exits a spinning die. Coupled with that volatility, there may be unpleasant vapor and odors which may raise concerns for people involved in the processing and production of fibers. That same volatility of the low molecular weight fraction will tend to lead to a blooming or surface imperfection on the finished fibers after they are drawn due to the pitting and cracking which may be caused as the low molecular weight species volatilize. An additional maintenance headache is created by the recondensation of these low molecular weight species within ventilation equipment when they come back down to the melting point after having been volatilized from the melt as it exits the forming device or die.
Within fiber production itself, other problems can occur due to the increased fraction of low molecular weight species. This includes significant die drooling or drip which is attributable to separation of low molecular weight species at the die outlet. Reduced viscosity is caused by the low molecular weight species. Along with drooling, there is also a tendency for the low molecular weight species to collect at the exit of an orifice or a die. During high speed processing this collected scale may break off from the die or spinnerette face becoming included at the surface of the fiber which is being formed. This "slub" becomes an imperfection in the fiber which will probably cause a break in the line. When such slubbing occurs, it is likely that the line must be shut down. The die will require either changing or cleaning prior to re-starting production.
Essentially, to gain advantage in processing these polyolefins, the end-use producer accepts the detriments caused by the presence of increased low molecular weight fractions when the polymer producer provides product which has been treated by the post-reactor oxidative scission process. In addition to creating some other handling difficulties for the producer of fibers or fibrous products, the polyolefin producer must add another fairly expensive step to the production process. This not only increases costs but also complicates the process of polyolefin resin production for the polymer producer.
In light of the complications caused for both the polymer producer and the end user of the polyolefin it would be useful and valuable to produce isotactic polyolefins as reactor-grade materials having a narrow molecular weight distribution yet still having the nominal molecular weight of the post-reactor oxidatively degraded products. Aside from removing an expensive and process-complicating step from the polymer production process, this would lead to numerous benefits for the end user, particularly the polyolefin fiber producer.
Of particular value to the fiber producer would be reduction or elimination of the smoking and odor problems, ventilation fouling, and surface blooming on finished fibers. Additionally, a polyolefin product having 1) a centered narrow molecular weight distribution, centered around the nominal molecular weight, coupled with 2) the handling characteristics of the oxidatively degraded polyolefins would reduce such processing problems including drooling and slubbing which would thereby enhance not only the productivity of the fiber producer's equipment but also product quality.
Coupled with the previously mentioned benefits, the fiber manufacturer would have an opportunity to lower the melt strength of the polymer even further than possible with oxidative degradation, thereby allowing reduced diameter fibers to be produced with less die swelling than otherwise might be experienced with polypropylene containing a significant fraction of molecules having higher molecular weight than the nominal molecular weight. Such a polymer product would yield benefits to both the polymer producer by eliminating the need for an expensive and complicated post-reactor process step and the fiber manufacturer by offering the potential for higher rates of production of finer fibers.
Kloos teaches, in "Dependence of Structure and Property of Melt Spun Polypropylene Fibers on Molecular Weight Distribution" published, at pages 6/1-10, by The Plastics and Rubber Institute's Fourth International Conference on Polypropylene Fibres and Textiles, a conference occuring in September 1987, that spinning of polypropylene improves with degradation of the polymer by peroxide. These findings were significant and led to the determination of a value, to which fiber and spinning characteristics were correlated, termed the "Degradation Ratio".
Branchesi and Balbi teach, in "Mechanical and Structural Properties of As-Spun Polypropylene Filaments in Relation to Resin Rheology" published, at pages 27/1-9, by the Plastics and Rubber Institute's International Conference, which occurred in November 1989, that degraded or "controlled rheology" polymers find use in high spinning speed operations.
Amos and Goldin reported, in "Creating Higher Performance Fibers With a Novel Additive Concentrate" published in International Fiber Journal, Oct. 19, 1993 pages 88-96, other additives improve fibers made from "visbroken" polymers or controlled rheology polymers. These reseachers accomplished their visbreaking, or reduction in melt viscosity, through the addition of liquid or low melting fractions added to the polyolefin resin from which fibers are to be made. This apparently provides a low molecular fraction which appears to not only improve the fiber making process but also appears to provide fiber webs of greater filtration efficiency than typically peroxide treated resins.
Canich in U.S. Pat. No. 5,026,798 teaches that resins produced by a monocyclopentadienyl-hereatom transition metal complex catalyst system can be used to make a variety of products including films and fibers. The polymers so produced apparently fall within the broad MWD range of somewhere between about 1.5 to about 15, as may be noted at Column 9, Line 53. Canich, unfortunately offers no guidance as to where, in this particular broad range of molecular weight distribution, the fibers which are so glibby mentioned may be produced economically. Further, Canich offers no direction as to the treatment of the resin for producing satisfactory fibers. Therefore, the off-hand remark regarding usefulness of that material for fiber formation is of little value in the enablement of fiber production.